This invention relates to a method of selectively depositing a metal film, and especially relates to the method realizing good electric contact with an underlying layer of the metal film and high selectivity of growth of the metal film.
High integration of semiconductor devices has been brought by miniaturizing composition elements such devices. However, in the manufacture process of semiconductor devices, various problems which are shown below have occurred. In wiring processes, for example, wiring widths have become small by reduction of a design rule in accordance with miniaturizing and the aspect ratio of contact hole (depth of contact hole/width of contact hole) for connecting an upper wiring layer and a lower wiring layer has been increased. For this reason, when forming an Al--Si--Cu alloy layer in a contact hole by using the usual sputtering method, it has become difficult to form a reliable wiring, because the wiring is either cracked or broken down in the bottom of the contact hole.
As a method of solving the problem resulting from the high aspect ratio of such contact hole, some methods propose forming wiring material by embedding the contact hole with conductive material and thereafter making it flat. As one of the methods, a selective chemical-vapor-deposition (CVD) technology which selectively forms a metal film such as tungsten (W) only in the contact is known. This technology is remarkable because a metal film can be grown up from the bottom of a contact hole, even if the contact hole is deep.
The selective CVD technology is precisely explained below.
An insulating film is formed on a semiconductor substrate, and if a contact hole for taking electric connection is opened by reactive-ion-etching (RIE), the RIE damage layer will form on the bottom of the contact hole. The reason why that this RIE damage layer is formed is as follows.
After applying a resist and transferring a desired pattern on the insulating film by the optical exposing method, an opening of the contact hole with use of a RIE method is performed by etching the regions of the insulating film not protected by the resist. For example, if the insulating film is in the case of a silicon oxide, an etching is performed using the compound gas including fluorine atoms such as CF.sub.4.
As plasma is used at this RIE process, electric or ion particles put onto the substrate. The residual object of CH system produced from the resist, and F or C driven from the plasma gas remain in the bottom of the contact hole. Furthermore, the resist that remained in the substrate surface is removed after opening the contact hole. Removing the resist is usually carried out by an electric discharge using an oxygen gas for ashing the resist with use of oxygen radical (O*). The bottom of the contact hole also oxidizes in this ashing-process. Therefore, the RIE damage layer is formed from RIE process and ashing process.
If RIE damage layer exists on the bottom of the contact hole, the metal film formed by the selective CVD method does not grow because the RIE damage layer may work as an insulating film. Therefore, the RIE damage layer needs to be removed before growth of the metal film.
There is a method of removing RIE damage layer by wet chemistry processing of rare hydrogen fluoride etc. After the wet chemistry processing, however, a substrate is washed by pure water and is dried in N.sub.2 atmosphere and is usually conveyed with exposure to the atmosphere to the apparatus for perform selection CVD. Therefore, a natural oxide layer grows to the surface of the substrate again in the process which is conveyed in the atmosphere, even though a pure metal surface or a pure semiconductor surface is exposed by the wet chemistry processing. In situations where that the natural oxide layer exists, an electrical property will be change for the worse since a metal film is grown through the natural oxide. Therefore, the method by wet chemistry processing has not resulted in practical use.
Thus, after removing RIE damage layer and exposing it the pure surfaces, a metal film must be formed on the pure surface without exposing to the atmosphere.
One example of such method is indicated in Japanese Patent Disclosure (kokai) No. 60-91631 (prior art 1). According to the method W is selectively formed after exposing to a gas plasma so as to remove RIE damage layer. The method is especially effective if the sputtering to the RIE damage layer with Ar ion generated from electric discharge of Ar gas is carried out.
However, the sputtering is carried out not only against the bottom of the contact hole but against the surface of the silicon oxide. The sputtering is the phenomenon that atoms with light atomic weight are firstly sputtered (selective sputtering). In the case of the silicon oxide layer an oxygen atom (O) is firstly sputtered, and at the surface after sputtering surplus silicon atoms which are not proper chemical amount of atoms are formed. This surplus Si forms a dangling bond and the selective deposition of a metal film cannot be realized.
The mechanism of the selective deposition of W film which is one of the metal films is shown in Ito et al. "Japanese Journal of Applied Phisics, 30 No. 7, page 1525-1529 (1991)" (prior art 2).
Namely, the essence of a selective deposition is that electrons move to WF6 absorbed from the surface of the substrate and start absorption and dissociating and thereby a seed layer is formed. The dangling bond of Si atom has a non-pair electron and tends to serve as an electronic grant object and therefore W is grown on the surface. For this reason, it is difficult to carry out the selective deposition of W film by the method of Japanese Patent Disclosure (kokai) No. 60-91631.
In view of this situation, a method of terminating the dangling bond produced on an insulating film is proposed in Japanese Patent Disclosure (kokai) No. 2-38568 (prior art 3). In this method, after a plasma etching of the processed substrate surface is carried out by Ar gas and so on, for clean up, a substrate exposed to a desired gas atmosphere and the dangling bond is terminated by O, N, F, or OH. Therefore, a good metal film is formed.
Moreover, a method of selective deposition of a metal film after etching a opening part of a contact hole formed in an almina film or SiO.sub.2 layer with use of gas plasma including chlorine such as BCl.sub.3 is also proposed in Japanese Patent Disclosure (kokai) No. 1-201938.
According to this method, since the insulating film formed at the surface of the contact hole can be removed without damaging the surface of the alumina film or SiO.sub.2 layer by using gas including chlorine, it is possible to reduce a contact resistance at the contact hole.
However, it has became clear to inventors that it is difficult to obtain the high selectivity and a good metal film even if these methods implement selective CVD.